The present invention relates to optical scanning and, more particularly, to a method and apparatus for optical scanning providing both the high resolution and high speed desired for applications such as non-impact printing and computer input/output.
Because from the theoretical standpoint optical scanning can provide great speed in information processing, much work has been undertaken to develop practical optical scanning systems for printing and/or reading information. This work has resulted in the development of systems which meet specific word processing or computer input/output criteria. In general, however, such systems are complex and expensive because of the extreme fabrication tolerances which must be met in order to provide both the high resolution and high speed typically required.
Most optical scanning systems use a rotating scanner of some sort which deflect a beam of optical radiation across the area it is desired be scanned. The difficulty with mechanically rotated optical scanners, though, is that fabrication tolerances and complexities escalate enormously, as faster scan rates (and consequently faster rotational speeds of the scanner) are sought. Achievable scanning rates are thereby limited as a practical matter.
Another problem associated with a rotating scanner is that as its deflecting surface or surfaces rotate to provide scanning, the focal point at which an image is focused traces a curved line or surface. Thus, in the absence of steps to change the point of focus as the deflecting surface rotates, the field at which the focused image will be provided, will be a curved surface. Some have attempted to correct for this curved focus field by providing a curved mechanism to support the medium which is to be scanned. In high speed printing or photographic imaging, though, it is necessary that the medium upon which the printing or image is being formed move rapidly through the image field. Because of the difficulty of providing rapid movement through a relatively uniformly curved space, most of those working in the field have found it necessary to turn to relatively costly and expensive optics to vary the imaging focal length of the scan and thereby provide a generally flat image field.
Acousto-optic deflectors are used in some scanning systems in place of mechanically rotated scanners. While generally faster scanning can be accomplished with an acousto-optic deflector, the resolution obtainable with such a scanner typically is significantly lower than that obtainable with a rotating scanner. Moreover, the deflection rate achievable with an acousto-optic deflector, although faster than that achievable with a rotating scanner, is also limited by the "cylindrical lens effect" (to be discussed in more detail below) unless relatively expensive optical elements are used for correction.
It can be seen from the above that scanning systems relying separately on either rotating scanners or acousto-optic deflectors are not optimum. Most of such scanners provide either high resolution or high speed. That is, if it is desired that a high resolution image be provided, it is at the sacrifice of speed. On the other hand, if high speed scanning is desired, it generally only can be achieved at the sacrifice of resolution. It is because of the desire to achieve both high resolution and high speed in a single application that most scanning systems are quite complex and expensive.